Twin robots learn to live and work with each other

A robot cell developed using two Mitsubishi Electric MELFA RV Series robots has used the latest robot control technology to ensure their movements are perfectly synchronised. The production cell in question prints and assembles over three million components per year, changing over seamlessly on-demand to assemble a range of different parts.

Most industrial robots working in close proximity to each other are programmed simply to avoid each other, this system however uses a Mitsubishi Electric iQ PLC (programmable logic controller) to control both robots in full synchronisation. Part of their movement is physically taught, part is programmed and an element of working together is left for them to learn how to achieve in the most efficient way.

Paul Judge of Mitsubishi Electric comments, “The advantage we have here is that the robot controllers don’t have to be standalone separate units. The robot control CPU (central processing unit) modules we used are actually mounted on the same backplane as the PLC, so there is no lag or communication hysteresis in the system, it operates and learns, in real time. It sounds highly advanced, which it is, at the same time it is a standard feature on this iQ PLC model.”

The project actually started at one of Mitsubishi Electric’s Robot Integration Seminars where an engineer from an authorised Mitsubishi Electric System Integrator was impressed by a demonstration of a robot working with a 3D vision system. His level of interest was such that immediately following the seminar a robot was loaned to his customer to put on trial. Paul Judge adds, “We have a number of robots that we send out to customers and integrators to trial systems with, it helps with building knowledge and familiarity for the engineers and the end user. We also find that, more often than not, the project ends with a sale.”

Technically each iQ PLC can support up to four separate processors, so could be configured with their own core CPU plus three robot controllers. The two-robot system is therefore well within the iQ control system’s capabilities. For this cell the two robot controllers were integrated into the iQ PLC rack along with some additional input and output cards and a couple of specific output modules used to interface to specialised sensors.

The project was based on standard Mitsubishi Electric automation components and even the robots were supplied from stock, which proved very cost effective. The robots are standard RV models with a 2kg payload capacity and six axis movement. They pick loose parts from a feed tray and manipulate them into different fitting, testing and printing stations. They also collaborate on dealing with reject parts.

The robots work in close cooperation with the cell’s vision system, the integration of their respective control systems being simple plug-and-play processes. The vision data is communicated to both robots simultaneously for maximum control clarity. The controllers have the capacity to communicate with up to seven vision systems, so can be used in very sophisticated multi-function systems if required. Production progress and cell activity can all be monitored on Mitsubishi Electric GOT 2000 HMI (human machine interface) panels installed as part of the control system.

Built-in temperature sensors make adjustments to compensate for hot and cold running conditions (and will shut down operations if pre-set limits are breached). The robots can be programmed from a simple to operate teaching box. Using this, the programmer walks the robot through the cycle of movements and functions, which are recorded in the controller’s memory and used as the basis for the operating program. Alternatively they can be programmed directly from a Mitsubishi Electric GOT 2000 HMI.

The RT ToolBox2 software has a 3D graphic display function which makes visualising the robot’s functions very easy and from which set point coordinates can be captured. The software also has a simple pick-and-mix system allowing various jobs or tasks to be readily selected as required.

The RV-2Fs have an arm geometry that gives maximum movement in every plane and extends the operating envelope of the robot in all directions. With the latest models the wrist joint is particularly strong and flexible. The RV-2F robots can also be mounted on turntables or linear slides so that they can move between two or more workstations, with the movement being managed by the robot controller.

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